INEXPENSIVE HEAVY METAL REMOVAL BY FOAM FLOTATION
Gregory Mclntyre, Research Assistant
Juan J. Rodriguez, Research Assistant
Edward L. Thackston, Professor
David J. Wilson, Professor
Department of Civil and Environmental Engineering
Vanderbilt University
Nashville, Tennessee 37235
A prime concern of the environmental engineer at the present time is the removal of
small concentrations of toxic substances from industrial wastewaters. Heavy metals are of
prime importance because the new pretreatment standards require dischargers to limit
heavy metals to very low residual concentrations, and the Environmental Protection Agency
estimates the metal-plating industry to be one of the industries hardest hit by the pretreatment standards.
Existing methods of heavy metal removal, such as lime precipitation, require large tanks
and space requirements, produce a wet, bulky sludge, and have long settling times. In order
to meet the 1984 BAT Regulations, polishing filters will probably also be required. This
will increase the cost even more.
Adsorbing colloid foam flotation as a way to remove heavy metals has been under investigation at Vanderbilt University for some years now. The process involves the addition of
a coagulant, usually ferric chloride or alum, to form a floe (usually positively charged). The
dissolved heavy metal is then adsorbed onto the floe particle and/or coprecipitated with it.
The addition of a surfactant of opposite charge, such as sodium lauryl sulfate (NLS), then
renders the floe particle hydrophobic and the floe (with adsorbed metal) is removed by
air flotation which produces rising bubbles of surfactant, with attached floe. When dealing
with dilute wastes, this process possesses some distinct advantages-low effluent metal
concentrations, rapid operation, low space requirements, production of small volumes of
sludge, flexibility of application to various metals at various scales, and, above all, low cost.
The work described in this paper involves the removal of copper and zinc, and mixtures
of copper and zinc, and chromium. The pilot plant studies have shown that effluent cop-
per(ll) concentrations in the range of 0.1 to 0.3 mg/1 and zinc levels around 0.5 mg/1 can be
routinely obtained under optimum conditions. For experiments done with a simulated
electroplating waste stream containing 20 mg/1 each of Cu(II), Zn(II) and Cr(IU), effluents
were produced which contained concentrations of Cu(II) less than 0.1 mg/l;Zn(H) less than
0.5 mg/1; and Cr(IH) less than 0.2 mg/1.
Earlier work involving a simulated lead waste was reported at the 35th Annual Purdue
Industrial Waste Conference [1] and in the WPCF Journal [2]. Wilson and Clark (3) also
published a lengthy summary review of adsorbing colloid foam flotation in 1978.
EQUIPMENT DESCRIPTION
The foam flotation pilot plant used in this work has been described in detail in earlier
papers [ 1,2]. Figure 1 shows a schematic diagram of the plant.
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